WSU Extension

Hortsense

Common Insects & Mites : Inchworms
(revision date: 4/30/2013)


Use Integrated Pest Management (IPM) for successful plant problem management.

Biology
Inchworms or measuring worms (sometimes called spanworms) belong to a family of moths called geometers and feed largely on ornamentals (e.g., rhododendron) and native trees and shrubs. There are many species and all types of plants can be affected. Some feed exclusively on conifers, while others only feed on deciduous trees and shrubs. Some species feed on both. Most of the native species are sporadic in their damage profile and are usually suppressed by natural enemies. One introduced species, the winter moth, has regularly caused serious damage to plants like maple and blueberry and requires management on a regular basis. It is named 'winter' moth due to its moth stage of development stretching from October to December. Eggs overwinter on host plants and larvae hatch fairly early in the spring, typically around late March. Their feeding damage is quite generic and without seeing them do the damage, it can be difficult to diagnose the pest that is responsible. It can resemble the damage caused by sawfly, earwig, cutworms, and loopers. Some inchworms feed at night, so they may not be present when looking at the damage.
Management Options

Non-Chemical Management
  • Picking the inchworms off the foliage and destroying them may be a useful alternative.
  • Encourage natural enemies of inchworms including birds and spiders.
  • Some inchworms are night feeders. Determine if they are doing the damage by going out after dark with a flashlight to find the pests.
Select non-chemical management options as your first choice!

Chemical Management
IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides

If nonchemical management options do not give satisfactory control, several insecticides and some biologicals are effective in managing these caterpillars. Make certain that the product that you purchase is registered for the target host and effective against the pest insect.

Images
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Caption: Winter moth larva (inchworm) and damage
Photo by: A.L. Antonelli
Caption: Winter moth adults
Photo by: A.L. Antonelli
Caption: Looper on rhododendron
Photo by: S.J. Collman